You can now set the skin of the npc using the texture value and signature (You can use https://mineskin.org/ to generate them)
Code (YAML):
npc-skin: # You can use a player name or a texture URL. texture: "https://s.namemc.com/i/d4ea5250d1997cb9.png" # Leave blank to use the texture from above. # You can use https://mineskin.org/ to generate the skin value and signature. value: "" signature: ""